Conventionally, there are heat exchangers which exchange heat between a first fluid and a second fluid, for example, heat exchangers which exchange heat between exhaust gas and cooling water in an internal combustion engine of a vehicle.
For example, a heat exchanger disclosed in Patent Literature 1 is housed in a case (outer tube), and exchanges heat between a first fluid (exhaust gas etc.) and a second fluid (cooling water etc.) as the first fluid is supplied to the case and the second fluid is supplied to the inside of the heat exchanger.